Radioactive tracers have been used in the petroleum industry to monitor the effectiveness of efforts to fracture earth formations and to cement casings in place in a well bore. Most often a single radioactive tracer is used and its distribution surveyed with a gross gamma ray counting instrument. However, there are many instances, such as in fracturing operations involving the fracturing of multiple zones or the injection of fracturing fluids in several stages, where it is desirable to use two or more tracers. To obtain more and better monitoring information, a different isotope could be used for each stage of the operation, for each zone, or for tagging the various solid and fluid components of the fracturing material.
It has always been possible to use two or more tracers with different half-lives, and to make multiple logging passes over a period of days or weeks so that different decay rates at different locations can be used to establish where each tracer was deposited. However, the development of gamma ray spectroscopic devices has made it possible to obtain the desired information in a single pass of the logging tool through the well bore by observing the specific gamma ray signatures of each different tracer. These tools measure the energies of the gamma rays which are emitted by radioactive tracers placed in the well and the surrounding formations and a multi-tracer survey log is obtained by deconvolving the gamma ray spectral data into contributions from each individual isotope as a function of depth.
When combinations of radioactive tracers are used in a well operation, care must be taken in selection of the tracers to facilitate the deconvolution of the recorded spectra for ascertaining the energy contributions of the individual tracers.